Abrasive grain and method for producing it, polishing tool and method for producing it, grinding wheel and method for producing it, and polishing apparatus

ABSTRACT

An abrasive grain including a porous particle material in which a large number of fine particles for cutting edges form gaps partly among them and bond loosely each other. The particles for the cutting edges are produced by growing primary particles in secondary particles, which are formed by cohesion of a large number of primary particles, with a heat treatment at a temperature of forming necks at bonding points among the primary particles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an abrasive grain to carry out finishmachining of hard and fragile materials, such as silicon, glass and soon, and a metal material, for example, steel, aluminum and so on, andfurther to a method for producing the abrasive grain, a polishing toolcomposing of the abrasive grain and having a long operation life, apolishing apparatus which has the polishing tool and a method forproducing the polishing tool, which contributes to a high quality and ahigh efficiency of a processing.

2. Description of the Prior Art

For a final finishing of a part, which is made of a silicon wafer, aglass disk, various hard and fragile materials, or metal materials, apolishing process using a loose-abrasive slurry has been widely adoptedbecause this process is easy to use a fine abrasive grain so that a finefinishing surface can be obtained easily. Moreover, a large amount ofabrasive slurry can also maintain stable processing characteristics ofthe final finishing by means of a stable supplying of the slurry.

However, this polishing processing, which uses the abrasive slurry,requires a large amount of slurry, and discharges a waste of the largeamount of slurry, so that it has a great impact on environmentconservation.

This polishing processing has also a limit for increasing a processingefficiency. Under such circumstances, a development of a fixed-abrasiveprocessing tool, which can produce an excellent finishing surface morethan the similar to that obtained by using the loose-abrasive slurry, isactively carried out in various fields.

For a finishing process with abrasive grain, it is an advantageous touse a fine abrasive grain in order to produce an excellent surfaceroughness, which is also normally true for the fixed-abrasive processingtool.

However, in order to produce an excellent surface such as a mirrorsurface, a contact between a bonding material bonding the abrasive grainand a backing and a work piece is caused by using the fixed-abrasiveprocessing tool in which a particle diameter is less than several μm,and also chips of the bonding material and the abrasive grains areaccumulated among the abrasives grains, and then clogging of theabrasive grains are caused.

As a result, not only the removal rate would be decreased, but also thesurface quality would be damaged, and in a worst case, it is notpossible to carry out the finishing process of the work piece by theabrasive grains.

Moreover, even though a method for controlling the contact between thebonding material of abrasive grains and the work piece is taken, thereis a problem for declining processing efficiency because a diameter ofeach of the abrasive grains is small.

On the other hand, when an abrasive grain in which a diameter of aparticle is large is selected to use, it is possible to improve theprocessing efficiency, but the quality of the surface is deteriorated,and it tends to be difficult to produce the mirror surface.

In order to solve these problems, the abrasive grain is powdered andthen the powder is agglomerated and the fixed-abrasive processing tool,which uses the agglomerated powder as the abrasive grain is proposed.(References: Japanese Patent Laid-Open Hei7-164324, Hei8-155840,2000-198073, 2000-237962, 2000-176842, and 2001-129764, Japanese Patentpublication Hei9-504235)

For this fixed-abrasive grain processing tool, the excellent surfaceroughness is provided by an action of the fine abrasive grain, and atthe same time, an improvement of the processing efficiency by theagglomerated abrasive grain can be accomplished.

However, it is still required that further improvements in theprocessing efficiency and the processing life should be achieved.Moreover, these proposed technologies do not focus on a bonding strengthamong the fine particles comprising the abrasive grain, so thatrequirements of improving the processing efficiency can hardly be met.

SUMMARY OF THE INVENTION

An object of the present invention is to solve problems of said priorart. In other words, the object of the present invention is to providean abrasive grain, which can maintain an extremely high processingefficiency for a long processing time without loosing a surfaceroughness, a long operating life of a polishing tool and a polishingapparatus, which uses the abrasive grain.

According to a result based on the inventor's repeated careful researchabout a fixed-abrasive processing tool, which uses agglomerated powderof a fine abrasive grain as the abrasive grain after granulating thefine abrasive grain related to said prior art, it has been proved thatbasically, a bonding strength among particles comprising the abrasivegrain is very important factor although it is influenced by a workpiece.

In the prior art, the bonding strength among the particles composing theagglomerated abrasive grain has not been noted and studied at all.

During grinding or polishing process, the abrasive grain is worn awaygradually and the top of it becomes flat. The flat or planarized surfaceof the abrasive grain or a substantially planarized surface acts as apolishing surface or a cutting edge of the work piece.

A high polishing efficiency is achieved when using the abrasive grainconsisting of sintered ceramic. However, this abrasive grain is too hardto use because it does not have gaps inside. As a result, big scratch isnewly generated on a surface of a work piece by the processing, and thenthe surface roughness is deteriorated.

On the other hand, when using the abrasive grain consisting of secondaryparticles, which are formed by cohesion or agglomeration of fine primaryparticles, a high polishing quality is achieved because a sort ofcutting edges are formed by the primary particles and the gaps. Thecutting edges are composed of particles acting to polish a work piece.

However, this abrasive grain consisting of the secondary particles,which are formed by the cohesion of fine primary particles, can not beachieved to the high polishing efficiency if the bond among the primaryparticles are not controlled. This abrasive grain also can not beachieved to a practical use of durability and an operating life.

At this point, the present inventors found out that in the abrasivegrain, which is formed by gathering the fine particles, it is possiblefor the abrasive grain to wear away gradually by controlling the bondingstrength among the fine particles.

According to this abrasive grain, new cutting edges are generatedconsistently, and it is possible to maintain the high processingefficiency for the work piece and an excellent processingcharacteristic, which can produce a high quality of a surfacecorresponding to nanometer, for a long processing time. As a result, thewear of the abrasive grain itself is also controlled, and the tool canbe used longer.

In other words, characteristics of the abrasive grain related to thepresent invention consist of a porous particle material. The porousmaterial consists of a large number of fine particles for the cuttingedges, which contain gaps between primary particles bonded to eachother. A large number of the fine particles for cutting edges areproduced by growing the primary particles into secondary particles,which are formed by the cohesion of a large number of the primaryparticles via heat treatment at a temperature sufficient to form necksat bonding points among the primary particles.

According to this abrasive grain, when the abrasive grain is actuallyused, at least one part, which is adjacent to the gaps between theparticles acts as the grinding surface. New surfaces for the grindingare projected on the processing surface while the parts making theoriginal grinding surfaces are lost by the wear.

Consequently, when the polishing and grinding processing are conductedthe cutting edges are always generated voluntarily on the abrasivegrain, and it makes easy to eliminate wastes of the polishing and thegrinding and maintains the excellent quality effectively. Therefore, itis possible to have stable processing for the long processing time.

It is desirable that strength of compression failure for the abrasivegrain is more than 1 MPa and less than 500 MPa.

If the strength of compression failure for the abrasive grain exceeds500 MPa, scratchs tend to be generated easily on the processing surfaceso that a possibility of lowering the quality of the processing surfacebecomes higher. On the other hand, if the strength of compressionfailure for the abrasive grain is lower than 1 MPa, there is apossibility that a pre-finished surface of work piece cannot beeliminated completely. In other words, the abrasive grain might not bedurable enough to accomplish polishing. Because the bonding strength ofthe particles for the cutting edges are too weak, so that the polishingand the grinding processing cannot be operated sufficiently, and hardwear is caused, then the processing efficiency declines. Moreover, agrinding burn tends to be generated easily when it is applied to agrinding wheel.

At this point, the grinding burn is generated by a contact between abond to fix the abrasive grain and the work piece because the grindingwheel does not have the projections required of the abrasive grain.Moreover, a tarnish of the polishing surface is generated since thepolishing processing of grinding is not operated normally, and thetemperature of the polishing surface goes up.

Accordingly, the wear of the abrasive grain itself and the degree of thefallout, which is generated with losing the part comprising the cuttingedges due to the wear of the particles for the cutting edges areoptimized so that fine surface quality can be maintained with processingvery effectively.

At the same time, the wear of abrasive grain can be controlled, so it ispossible for the abrasive grain to keep a good balance betweenprocessing efficiency, the processing quality, and the longer operatinglife. Therefore, it is possible for the polishing tool, which has theabrasive grain, to have the long operating life.

It is more desirable that the strength of compression failure for theabrasive grain is more than 20 MPa and less than 300 MPa.

By using this abrasive grain, it is possible to conduct the processingfurther effectively with maintaining the higher quality of theprocessing surface. It is also possible to control the wear abrasion ofabrasive grain effectively, and have longer operating life for thepolishing tool having the abrasive grain.

It is desirable that a pores specific surface area of the abrasive grainis more than 18000 cm²/cm³ and less than 700000 cm²/cm³.

If the pores specific surface area is less than 18000 cm²/cm³, thescratch of the processing surface tends to be generated easily so thatthe possibility of deteriorating the surface quality of surface becomeshigher.

On the other hand, if the pores specific surface area is bigger than700000 cm²/cm³, since the bonding strength among the particles for thecutting edges is too weak, the polishing and the grinding processingcannot be operated sufficiently. As a result, it has a possibility thatthe pre-finished surface of work piece cannot be eliminated completelybecause hard wear of the abrasive grain itself is caused, and theprocessing efficiency extremely declines. Moreover, the grinding burntends to be generated easily when the area is applied to the grindingwheel.

Accordingly, the wear of the abrasive grain itself and the degree of thefallout, generated by losing the part comprising the cutting edges(grinding surface) due to the wear of the particles for the cuttingedges (grinding surface), are optimized to maintain the fine surfacequality with processing very effectively, and at the same time, the wearof abrasive grain can be controlled. Therefore, it is possible for theabrasive grain to keep a good balance between processing efficiency, theprocessing quality, and long operating life. It can make the polishingtool comprising this abrasive grain operate longer.

It is more desirable that the pores specific surface area is more than100000 cm²/cm³ and less than 300000 cm²/cm³.

By using this abrasive grain, it is possible to conduct the processingfurther effectively with maintaining the high quality of the processingsurface. At the same time, the polishing tool, which has this abrasivegrain, can be operated longer by controlling the wear of the abrasivegrain effectively.

Moreover, it is desirable that an average particle diameter of theparticles for the cutting edges of the abrasive grain should be smallerthan 5 μm.

If the average diameter of the particles for the cutting edges of theabrasive grain exceeds 5 μm, the scratch is generated on the processingsurface, and quality of the processing tends to be declined. It is anundesirable situation. In order to make the average diameter of thearticles for the cutting edges less than 5 μm, control of the heattreatment condition can be used to regulate the bonding process.

By using this abrasive grain, the excellent quality of surface isdefinitely received.

Moreover, if a binder, which bonds the particles for the cutting edgesto the abrasive grain, is not used, new particles for the cutting edgesare projected automatically when losing the part making the cuttingedges (grinding surface) by the wear. Accordingly, insufficiency in thegrinding surface due to the presence of a binder on the new cuttingedges can be prevented.

Problems for the processing quality such as crushing, clogging, residueof the binder, generation of the scratch by attaching the waste of thepolishing to the binder, and so on can be avoided.

This excellent abrasive grain can be produced by means of following aproducing method according to the present invention.

Consequently, the present invention of the producing method for theabrasive grain comprises a process for producing the secondary particlesby cohesion of a large number of the primary particles, and a processfor producing the abrasive grain consisting of the porous particlematerial in which the particles for the cutting edges are produced bygrowing the primary particles with heat treatment at the temperaturesufficient to form necks at the bonding points between primaryparticles. Thus, the primary particles are made into secondaryparticles.

According to this present invention of producing method for the abrasivegrain, when the produced abrasive grain is used actually, the partadjacent to the gaps between the particles acts as the grinding surface,and new particles for the cutting edges are projected on the processingsurface automatically while the part making the cutting edges is lost bythe wear of the particles.

Therefore, by using the produced abrasive grain, when polishing andgrinding processing, the cutting edges (grinding surfaces) are alwaysproduced automatically for the abrasive grain, improving efficiency andquality. Moreover it is possible to have stable processing for the longprocessing time.

It is desirable for the heat treatment of this producing method to beoperated under the requirement that the average diameter of theparticles for the cutting edges, which are formed by this method, issmaller than 5 μm.

If the average diameter of the particles for the cutting edges exceeds 5μm, an undesirable scratch may be generated on the processing surface,and the processing quality tends to be deteriorated. If the polishinggrinding tool is produced by use of this abrasive grain, it is possibleto maintain excellent quality and have stable and effective processingfor a long processing time.

It is desirable for the polishing tool that the abrasive grain isexposed on a surface of the polishing tool.

Use of this polishing tool can prevent a deterioration of the processingquality occuring between the abrasive grains or between the abrasivegrain and binder, which fixes the abrasive grain and bonds the backing.

At this point, more than one kind of material, such as, for example,resin, ceramic, and metals can be chosen to use as the binder. Moreover,for example, it is possible to use a ceramic precursor to produceceramic after the heat treatment.

The polishing tool can be also selected from a lapping film, a polishingpad grinding cloth, and a grinding wheel.

By using a lapping film, the polishing pad, and the grinding wheel forgrinding, it is possible to bring out effects, which provide the highprocessing quality.

When the lapping film is produced in accordance with the producingmethod of this invention, the effects of abrasive grain can be achieved,and at the same time, the lapping film can be produced with a low cost.In other words, it is possible for the polishing tool to processpolishing with the relatively low cost even though the condition of thepolishing tool is disposable or similar to disposable.

When producing the polishing pad in accordance with this invention, theeffects of the abrasive grain can be achieved sufficiently, and at thesame time, it is possible to use the polishing pad instead of using asurface platen for a traditional polishing apparatus and a lappingmachine as the polishing tool. Moreover, unlike a lapping film, it ispossible for the polishing pad to be used for the long processing timesince new abrasive grain is generated on the surface of the polishingpad after being conditioned.

Therefore, it is possible for the work piece to be finished with theexcellent quality of the surface effectively. Moreover, the longoperating life of this polishing tool contributes to lower tool cost anda reduction in effort a task to change the polishing tool for workers.

Moreover, by adding the abrasive grain of this invention into thegrinding wheel it is possible for the work piece to be finished with thestable excellent surface quality without generating grinding burn at thetime of the polishing and grinding processing.

When producing the lapping film in accordance with the producing methodof this invention, it is desirable that a thickness of the binder, whichfixes the abrasive grain on a backing film, is smaller than a maximumdiameter of the abrasive grain.

By using this lapping film, the deterioration of polishing quality,which is caused by a contact of the binder to the polishing surface, canbe prevented, and at least the minimum amount of the projection of theabrasive grain is guaranteed.

It is also desirable that a content ratio of the abrasive grain at apart comprising the abrasive grain of the polishing tool is more than 5percent in volume and less than 90 percent in volume.

If the content ratio of the abrasive grain is less than 5 percent involume, it has a possibility that an the abrasive grain will not beeffective. On the other hand, if the content ratio of the abrasive grainexceeds 90 percent in volume, strength for maintaining the abrasivegrain declines and it cannot be used as the polishing tool because anamount of the bonding material for the abrasive grain is too low.

By using this polishing tool, it is possible to have the excellentquality with the high efficiency.

Moreover, the producing method of the polishing tool for this inventionhas a process for producing the secondary particles by the cohesion of alarge number of the primary particles; a process for producing theabrasive grain consisting of the porous particle material in which theparticles with the cutting edges are produced by bonding together theprimary particles with heat treatment at a temperature sufficient toform necks at bonding points between the primary particles and thesecondary particles, and many of the primary particles used as cuttingedges form gaps between connection points around the secondary particlesand bond to each other.

By using the producing method of the polishing tool, it is possible toproduce easily a long operating life polishing tool, which can polishextremely effectively while maintaining excellent quality.

Moreover, for the process of fixing the abrasive grain on the backing,it is possible to produce a polishing tool, which satisfies a demandedheat resistance, strength and so on, by using more than one kind ofbinders selected among resin, ceramic, and metal. It is also possible toimprove adhesiveness with the binder by a modification treatment for thesurface of the abrasive grain, which is used for this process.

A fixing method includes applying a mixture consisting of such as thebinder and the abrasive grain to the backing by using for examplewire-bar-coater, gravure-coater, reverse-roll-coater, knife-coater, andso on.

Moreover, in the process of fixing the abrasive grain, which is producedby the producing method of this polishing tool to the backing, it ispossible to produce the long operating life polishing tool by fixing theabrasive grain on the backing with a reinforcing material.

It is possible to use inorganic fiber such as metal powder, carbonfiber, glass fiber, and so on, organic fiber such as polyacrylonitrile,and so on, and metal fiber, and so on as the reinforcing material.

When using the fiber, it is possible to use an appropriate length of thefiber such as chopped fiber, milled fiber, and so on depending on need.It is also possible to improve the adhesiveness with the binder by themodification treatment for the surface of these fibers. It is alsopossible to use various types of whisker beside identified above as thereinforcement material.

Moreover, it is possible to produce the lapping film and the polishingpad by using the producing method of this polishing tool.

The producing method of the polishing tool related to the presentinvention has the process for producing the secondary particles by thecohesion of a large number of the primary particles; the process forproducing the abrasive grain consisting of the porous particle materialin which the secondary particles are produced by bonding together theprimary particles with heat treatment to form a neck at bonding pointsbetween the primary particles and the growing secondary particles. Theprocess involves a large number of the primary particles which form gapsbetween each other as they bond together to form the secondaryparticles. In other words, the primary particles stick out from thenewly formed secondary particle an provide grinding surfaces. Anotheraspect of the process for producing an abrasive grain mixture materialinvolves mixing or agitating the binder bonding the abrasive grain andthe abrasive grain and a process for producing a grinding wheel byforming the abrasive grain mixture material.

In accordance with the producing method of the grinding wheel, thegrinding wheel, which can polish extremely effectively while maintainingexcellent quality.

At this point, it is possible to use more than one kind of materialsamong resin, ceramic, and metal as the binder. Moreover, after using theceramic precursor, it is possible to produce the ceramic by the heattreatment.

By using the producing method of the grinding wheel, it is possible toimprove rigidity and stiffness by adding the reinforcing materials inthe process of producing the abrasive grain mixture material produced bymixing or agitating the bonding material, which bonds the abrasive grainand the abrasive grain. Moreover, the long operating life grinding wheelcan be produced.

Moreover, the reinforcement material includes organic fiber, inorganicfiber, and metal fiber is used, and it is possible to use an appropriatelength of the fiber such as a chopped fiber, a milled fiber, and so ondepending on need.

It is also possible to improve the adhesiveness with the binder by themodification treatment of the surface for these fibers. It is alsopossible to use various types of whisker as the reinforcing material.

Moreover, it is possible to improve the adhesiveness with the binder bythe modification treatment of surface for these reinforcement materials.

The polishing tool of the present invention can be attached to thepolishing apparatus.

This polishing apparatus has high polishing efficiency, high quality ofpolishing, and long operating life for the polishing tool, which canreduce the time of changing the polishing tool. It is possible for thepolishing tool to have the high processing efficiency, the highprocessing quality, and the long operating life.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relationship between a load of a strengthtest of compression failure for an abrasive grain and a displacement ofpressure.

FIG. 2 is a photograph of a scanning electron microscope (SEM) showing awhole image of second particle.

FIG. 3 is a photograph of a scanning electron microscope (SEM) showingsecond particles, which are enlarged partly before a heat treatment.

FIG. 4 is a photograph of a scanning electron microscope (SEM) showingabrasive grains, which are enlarged partly after a heat treatment.

FIG. 5 is an enlarged photograph of a glass disk surface beforepolishing of embodiment 1

FIG. 6 is a chart showing a measurement result of a surface roughnessfor the glass disk surface before a grinding of embodiment 1.

FIG. 7 is an enlarged photograph of the glass disk surface afterpolishing grinding in embodiment 1.

FIG. 8 is a chart showing a measurement result of the surface roughnessof the glass disk surface after polishing in embodiment 1.

FIG. 9 is an enlarged photograph showing a condition of abrasive grainson a surface of a lapping film after using as polishing in embodiment 1.

FIG. 10 is an enlarged photograph of a glass disk surface beforepolishing in comparative example 1.

FIG. 11 is a chart showing a measurement result of a surface roughnessof the glass disk surface before polishing in comparative example 1.

FIG. 12 is an enlarged photograph of the glass disk surface afterpolishing in comparative example 1.

FIG. 13 is a chart showing a measurement result of the surface roughnessof the glass disk surface after polishing in comparative example 1.

FIG. 14 is an enlarged photograph showing a condition of abrasive grainson a surface of a lapping film after using as polishing in comparativeexample 1.

FIG. 15 is an enlarged photograph of a glass disk surface afterpolishing in comparative example 2.

FIG. 16 is a chart showing a measurement result of a surface roughnessof the glass disk surface after polishing in comparative example 2.

FIG. 17 is a graph examining a relationship among strength ofcompression failure for an abrasive grain, a surface roughness and aprocessing efficiency for a lapping film, which comprises various typesof the strength of compression failure for the abrasive grain.

FIG. 18 is a graph examining a relationship among a pores specificsurface area, a surface roughness, and a processing efficiency for agrinding film, which comprises various types of strength of compressionfailure for an abrasive grain.

FIG. 19 is a view showing an example of a polishing apparatus forprocessing silicon installing a grinding wheel, which comprises anabrasive grain based on the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An abrasive grain according to the present invention consists of aporous particle material. The porous particle material consists of alarge number of primary particles which bond together to form secondaryparticles, but stick out from the secondary particle to form gapsbetween each other. The primary particles bond to each other vianeck-shaped bonds. A large number of the secondary particles for thecutting edges (for grinding) are produced by agglomerating primaryparticles into secondary particles with heat treatment at a temperaturesufficient to form neck-shaped bonds at bonding points among the primaryparticles. The secondary particles are formed by cohesion of a largenumber of the primary particles.

The abrasive grain of the present invention is different from atraditional agglomerative abrasive grain in which the fine primaryparticles were simply agglutinated to form the secondly particles asmentioned in the references. The abrasive grain of the present inventionmaintains a structure of the porous particle material in which a largenumber of the secondary particles used for the cutting edges (andgrinding) contain gaps which are spaces between primary particles bondedtogether.

According to the abrasive grain of the present invention, strength ofits bonding is stronger compared with strength of its bonding among theprimary particles in the traditional grain because necks made of theparticle material are formed at the bonding points. The necks are formedby bonding the primary particles with heat treatment to form secondaryparticles. As a result, it is possible for the abrasive grain of thepresent invention to use for a long processing time with maintaining thepolishing efficiency and the polishing quality.

The particles with cutting edges, in one non-limiting embodiment of thepresent invention, are particles formed from primary particles bondedtogether to form secondary particles. The bonding occurs by cohering theprimary particles via heat treatment. At least one part of the secondaryparticles functions as the cutting edges for a work piece when using itfor the polishing and grinding.

Moreover, after the heat treatment for growing the secondary particles,the secondary particles are not only grown by a movement of asubstances, which builds the particles, but also the bonding parts ofamong the primary particles becomes thicker by the movement of thesubstances, which forms the particles, then becomes a smooth curvedsurface without a discontinuity. That is, after the heat treatment isconducted as growing the primary particles; it becomes so called a“neck”, which is necked as a hyperboloid of one sheet (hand drum shape).

“2.3 Mechanism of Substantial Movement and Model of sinter” in“Technical collection of ceramic material” issued from IndustrialTechnical Center, Ltd, (issue of S54, April 10, No. 1) describes indetail about growing the primary particles and forming the “neck” bymovement of the substance at the heat treatment.

The abrasive grain of the present invention does not include a binderbecause it is bonded by means of the substances composing the particlesfor the cutting edges among the particles for the cutting edges.

As a result, new cutting edges are automatically projected on theprocessing surfaces.

Therefore, it is possible to solve a problem that the amount of theprojection from the binder becomes insufficient when forming theabrasive grain by using the binder.

Moreover, by using the abrasive grain of the present intervention it ispossible to solve problems such as occurrence of defects, and so on forthe processing quality including crushing, clogging, and generation ofscratch by residue of the binder, adhesion of the waste to the binder,and so on.

However, when forming the secondary particles by cohering a large numberof the primary particles, it is possible to use binder, which isdissolved completely by the heat treatment such as an oxygenation, adecomposition, an evaporation, or the like, and for example, binderconsisting of an organic material. When using these kinds of binder, itis possible to solve the problem as mentioned above because the binderdoes not remain when using the abrasive grain.

The strength of bonding the particles for the cutting edges is increasedby forming of the necks among the particles at the bonding points. As aresult, combined with growing the particles itself, it is possible toproduce an abrasive grain at a high processing efficiency rate, the highquality of the processing, and long processing life.

For materials composing the primary particles of the present invention,it is appropriate to use a hard inorganic material including acharacteristic that the substances composing the particles are moved andgrown by the heat treatment, and can be adopted as the abrasive grain(can be used as the abrasive grain). These substances include silica,ceria, cubic nitriding boron (cBN), alumina, silicon carbide, zirconiumoxide, and so on. It is desirable to use the primary particles in whichan average diameter of the particles is smaller than 5 μm.

The secondary particles of the present invention are a body of cohesionconsisting of a large number of fine primary particles. A method forproducing these secondary particles by the cohesion a large number offine primary particles each other includes a splay drier (Generally,from size of 1 μm to 300 μm of the secondary particles can be produced.When a granulometry is not displayed sharply, a classification processis carried out), a sol-gel method, a freeze-dry method, a solvent drymethod, and so on, to which the solvent is used together.

There is another method, which uses a thermal decomposition and asolid-phase reaction. Moreover, as a method for forming from gas, it ispossible to use evaporation and cohesion, vapor-phase decomposition,other gas phase reactions, and so on.

The primary particles in the secondary particles are grown by the heattreatment for the secondary particles, which are produced by thesemethods.

The heat treatment of the present invention is conducted under thecondition (temperature and time) of growing the primary particles intothe secondary particles, which are formed by the cohesion a large numberof fine primary particles.

The condition of the heat treatment is chosen appropriately by thesubstance, which composes the primary particles. Generally, thetemperature is chosen under the condition that the heat treatment can befinished within 10 minutes to several hours. If the time of the heattreatment is too long, it becomes hard to control the particles, and theparticles are sintered such as the abrasive grain made of normalceramic. Even though the particles with the cutting edges are notsintered, the situation is similar to being sintered essentially becausethe secondary particles become too big. In this case, the effects of thepresent invention cannot be achieved.

We have conducted examinations of the heat treatment in advance based onseveral different temperatures and times, and then we have examined astructure of inside the particles after the heat treatment by anelectron microscope and so on, and then we have found out the conditionthat the bonding part among the particles for the cutting edges becomethe smooth curved surface without the discontinuity.

In other words, we have found out a condition that the primary particlesbecome the hyperboloid of one sheet as a proof of growing the primaryparticles, and it is a condition of a range, which becomes so called a“neck”. It is also the condition of the range maintaining the porousmaterials in which a large number of secondary particles contain gapsbetween the primary particles bonded to each other which make up thesecondary particle.

These temperatures and times are different based on the materials, butin case of the hard inorganic materials as explained above, thetemperature is about from 500 to 1600° C., and the time is from severalminutes to 24 hours. In this case, it is possible to conduct the heattreatment with pressurizing.

However, when the particles for the cutting edges, which compose theabrasive grain becomes too big by growing, it has the possibility thatthe effects of the present invention can not be achieved because of offbalance. Therefore, it is desirable that the heat treatment is conductedby the condition in which the average particle diameter of particles forthe cutting edges within smaller than 5 μm of the abrasive grain.

Moreover, it is preferable to conduct the heat treatment under thecondition that the strength of compression failure for producing theabrasive grain is more than 1 MPa and less than 500 MPa or the poresspecific surface area is more than the 18000 cm²/cm³ and less than700000 cm²/cm³.

In accordance with the abrasive grain produced by this condition, it ispossible to polish the high processing surface quality more effectivelytogether with controlling the wear of the abrasive grain.

It is also possible to use the polishing tool comprising this abrasivegrain for longer time. Moreover, it is possible to save time forchanging the polishing tool, and also lower the cost, which is requiredto change the polishing tool.

TESTING EXAMPLE

Concrete testing examples about the abrasive grain of the presentinvention is explained as follows.

However, the present invention is not only limited for these testingexamples, it is possible to implement in various ways within the rangein which the testing example does not deviate from the essential featureof the present invention.

Moreover the average particle diameter is measured by a dry system usingLaser Diffraction made of Horiba, Ltd. and Scattering GranulometryMeasuring Device LA-920, and the particle diameter of the abrasive grainat 50 percent of frequency accumulation is decided as the averageparticle diameter (normally called median diameter).

A test for the strength of compression failure is also carried out byMicro Compression Testing Machine, MCTM500PC made of Shimdzu Corporationbased on the report of Hiramatu, Oka, and Kiyama (Japan Mining IndustryMagazine 81,1024 (1956)).

The secondary particles are compressed by using a flat indentator underthe test conditions of the test load from 10 to 1000 mN, load speed0.446 mN/sec, and then the strength in which the secondary particles aredestroyed by compression was measured. FIG. 1 shows a relationshipbetween a compression displacement at this point and a load as a modelat this point.

The strength of compression failure, T reads the load value at thecurved line bending part Q inside the dotted line circle, and thencalculated by this value.

On the other hand an evaluation of a surface roughness for a processingsurface was conducted by using Form Talysurf S4C made of Taylor-HobsonLtd.

Moreover, the fine pores specific surface area of the present inventionis multiplied value of BET specific surface, which was measured byabsorption and de-sorption of BET one point method for relative pressure0.3 of nitrogen gas and density of a material composing the abrasivegrain.

Testing Example 1

Slurry was produced by adding water (water system binder for examplemixture of polyvinyl alcohol water is available to use) into ultra finezirconium oxide (ZrO₂) powder as the primary particles consisting of theparticle diameter from 50 nm to 60 nm. After the spraying the slurry bythe spray dryer, secondary particles α, which have 50 μm of the averageparticle diameter, are produced. The strength of compression failure forthese secondary particles α were 0.47 MPa. FIG. 2 and FIG. 3 shows awhole scanning electro microscope (SEM) photograph of the secondaryparticles and partly enlarged the scanning electro microscope (SEM)photograph.

The heat treatment for these secondary particles a was conducted by useof an electronic furnace. The polyvinyl alcohol, which was used as thebinder when forming the secondary particles, is eliminated completely bythis heat treatment.

In accordance with previously examined condition, the temperature of theheat treatment and the time of the heat treatment were adjusted tobecome the inside particles within the porous particle material, whichworks as the particles for the cutting edges when using as the abrasivegrain, smaller than 5 μm.

FIG. 4 shows a partly enlarged SEM photograph (the same magnification asFIG. 3) about an example of the porous particle material (abrasive grainβ) consisting of this zirconium oxide in which the heat treatment wasconducted by appropriate condition.

According to FIG. 4, the porous particle material (abrasive grain β),which works as the particles for the cutting edges when using it as theabrasive grain, is obviously growing bigger than the primary particlesshown in the FIG. 3, and the bond among the particles become the neckedhyperboloid of one sheet so called the “neck”.

It is also possible to identify that a large number of secondaryparticles contain gaps between primary particles bonded to each other.Moreover, when the heat treatment is conducted, if the time of the heattreatment is too long or the temperature of the heat treatment is toohigh, the primary particles are completely sintered, and then becomealmost solid.

The abrasive grain β related to the present invention, which wasproduced by the strength of compression failure 92.6 MP and the averageparticle diameter 50 μm, is mixed with liquid urethane resin to becomethe volume ratio of the particles is 35 percent of its volume.

Moreover, a mixture was produced by mixture and agitation of theabrasive grain and resin for ten minutes with an agitator afteradjusting solution viscosity by adding the solvent. The agitation wasconducted by a room temperature and 50 rpm of its revolution speed inwhich the speed does not destroy the abrasive grain.

The mixture was applied on the backing (PET film of about 75 μmthickness) by using the wire-bar-coater. Thereafter, it was dried forone hour in a constant temperature tank, which keeps the temperature of60° C., and then a lapping film A as the polishing tool was produced.

Maximum thickness of a produced application layer (a part of comprisingthe abrasive grain) becomes almost same as maximum diameter of theabrasive grain related to the present invention having the granulometry(By using the solvent, it becomes easier to thin the thickness of thebinder's layer).

The lapping film A produced by this method was attached to a lappingplaten, then after processing (processing condition: 60 rpm of platenrevolution speed, 46 kPa of processing pressure) an optics glass disk(borosilicate crown glass (suitable for BK7)), which Ry was adjusted to2 μm, a mirror surface without a scratch in which the maximum height forroughness Ry is less than 30 nm (nanometer) was produced in two minutes.

After that processing 20 sheets of the same glass disks under the samecondition were processed continuously, the processing efficiency and theprocessing surface roughness were barely deteriorated. At this point,FIG. 5 shows enlarged surface photograph GD of the glass disk before theprocessing. FIG. 6 shows the measurement result (chart) for the surfaceroughness of the glass disk. Moreover, FIG. 7 shows the enlarged surfacephotograph GD' (the same magnification as FIG. 5) after processing, andFIG. 8 shows measurement result for the surface roughness.

It is possible to see that irregularity, which was remained before theprocessing, was almost eliminated after the processing, and it becomesthe mirror surface. The condition of the wear of the abrasive grain onthe lapping film A after processing 10 sheets of glass disks wasexamined. FIG. 9 shows a condition of a surface A′.

According to FIG. 9, it is possible to see that the abrasive grain doesnot have big damage and fallout from the backing by progressing the weargradually along with progress of the processing because the bondingstrength among the particles for the cutting edges are appropriate.

Abrasive grain γ and δ of comparative examples 1, 2 were produced by thesame method, which produced the abrasive grain β by using the secondaryparticles α of the testing example 1. But the heat treatment conditionwas changed.

The strength of compression failure for the abrasive grain γ was 0.6MPa, and the pores specific surface area was 1000000 cm²/cm³. Thestrength of compression failure of the abrasive grain δ was 613 MPa, andthe pores specific surface area was 3000 cm²/cm³. The average particlediameter of these abrasive grain γ and δ were 50 μm for the bothabrasive grains.

Examination of each abrasive grain γ and δ, which was conducted by thescanning electron microscope, did not show a formation of the “neck” at0.6 MPa of the strength of compression failure of the abrasive grain δ.In other words, it was found that the enough heat treatment was notconducted, so that the primary particles were not growing.

On the other hand, 613 MPa of the strength of compression failure forabrasive grain δ did not have the structure that a large number of fineparticles for the cutting edges contain gaps between primary particlesbonded to each other and it became almost absolute sintered compact.

A lapping film B (the strength of compression failure of abrasive grainγ: 0.6 MPa, comparative example 1) and a lapping film C (the strength ofcompression failure strength of abrasive grain δ: 613 MPa, comparativeexample 2) were produced under the same method as producing the lappingfilm A by using each of these two abrasive grains γ and δ.

These lapping films B and C were attached to the lapping platensrespectively, and then processed BK7 optics glass disk in which themaximum height for the surface roughness Ry becomes 2 μm. For thelapping film B, the processing was conducted for 20 minutes, but themaximum height for the surface roughness Ry only achieved to 1.275 μm.

When conducting the same processing test by using the lapping filmproduced by using the second abrasive grain α (strength of compressionfailure of 0.47 MPa) in which the heat treatment was not done, theprocessing efficiency was lower than the result of the lapping film B,and the processing surface before polishing could not be improved.

FIG. 10 shows an enlarged photograph GD 1 of the surface of the glassdisk before the processing. FIG. 11 shows the measurement result of itssurface roughness. Moreover, FIG. 12 shows an enlarged photograph GD 1′(expansion magnification is same as FIG. 10) of the surface after 20minutes processing, and FIG. 13 shows the measurement result of itssurface roughness.

FIG. 14 shows the condition of the abrasive grain on lapping film B′after using as the processing.

These results show that even though the surface roughness of the glassdisk was improved a little by polishing for 20 minutes using the lappingfilm B, but the pre-finished surface of the glass disk was not removedcompletely because the hard abrasion of abrasive grain is generated(Reference of FIG. 14).

On the other hand, when using the lapping film C, a big scratch wasgenerated newly by processing the surface of the glass disk surfacebecause of using the abrasive grain, which became the absolute sinteredcompact bonding completely among the primary particles. In other words,the maximum height for surface roughness Ry was rather deteriorated as2.7228 μm (Scratch SK can be seen in the surface expanded picture ofFIG. 15).

An appearance of the scratch SK is recognized from the measurementresult of the surface roughness (FIG. 16) on the processing surfaceafter polishing. On the other hand, the abrasive grain was hardly wornaway, and the planarization of the edges for the abrasive grain was notseen by the observation of the microscope.

Lapping films comprising the abrasive grains, which have several kindsof the strength of compression failure, are produced in the same way.FIG. 17 shows a result of relationship among the strength of compressionfailure, the surface roughness (sign: □), and the processing efficiency(sign: ⋄).

In FIG. 17, a vertical axis of processing efficiency relatively showsamount of polishing per unit time, and the processing efficiency becomeshigher as reaching the upper part over the middle part of the graph.

According to FIG. 17, it is shown that if the strength of compressionfailure is too small (the strength of compression failure: less than 1MPa), in other words, if the bonding strength of the particles for thecutting edges are too weak, the surface roughness can not be improvedmuch because the processing efficiency is low, and a destruction of theabrasive grain is proceeded by processing pressure, so that theprocessing surface before the polishing can not be eliminatedcompletely.

Meanwhile, if the strength of compression failure is too strong, forexample, in case of the almost absolute sintered compact (the strengthof compression failure: 613 MPa), the processing efficiency becomesextremely high. However, on the other hand, the quality of theprocessing surface is highly deteriorated. In this way, only theabrasive grain related to the present invention, which comprises theappropriate bonding strength of the particles for the cutting bladeedges, could achieve the high quality of the processing surface (mirrorsurface) very effectively. At that time, it was found that the wear ofthe abrasive grain is controlled, and the polishing tool can operatelonger.

About the abrasive grains comprising the various strength of compressionfailure for the abrasive grain shown in FIG. 17, FIG. 18 shows arelationship between the fine pores specific surface area, which is aparameter showing inside structure of the abrasive grain, and theprocessing efficiency. At this time, the fine pores specific surfacearea is different from a normal specific surface area (unit is ┌cm²/g┘or ┌m²/g┘ and so on). In other words, the fine pores specific surfacearea is a parameter, which shows a difference of the inside structureprominently, because the fine pores specific surface area excludes anaffect based on a difference of a specific gravity of a material.

In FIG. 18, the processing efficiency of the vertical axis relativelyshows the amount of polishing per unit time, and the processingefficiency goes higher as reaching upper part over middle part of thegraph.

According to FIG. 18, it is shown that if the fine pores specificsurface area is too big (the pores specific surface area: over 700000cm²/cm³), in other words, if the bonding strength among the particlesfor the cutting edges is too weak, the processing efficiency is lowered.

The surface roughness cannot be improved much also because thedestruction of the abrasive grain by the processing pressure isprogressed and the pre-finished processing surface is not removedcompletely.

Meanwhile, if the fine pores specific surface area is too small, thestructure that a large number of fine particles for the cutting edgescontain gaps between primary particles bonded to each other, is lost.

For example, when using the almost absolute sintered compact (the poresspecific surface area: less than 5000 cm²/cm³), the processingefficiency is extremely improved. However, on the other hand, when usingthe almost absolute sintered compact, the quality of processing surfaceis highly deteriorated.

In this way, the only abrasive grain related to the present invention,which comprises the bonding strength among the particles for the cuttingedges and the specific structure, could achieve the high quality of theprocessing surface (mirror surface) very effectively. At this point, itwas found that the wear of the abrasive grain is controlled, and thepolishing tool can operate longer.

Testing Example 2

Silica abrasive grain, which consists of colloidal silica and diameterof the primary particle (average particle diameter of 50 nm) is coheredto become the average particle diameter 30 μm by the sol-gel method.Then the produced silica powder is dried and secondary particles ε wereproduced by eliminating water and organic solvent, which are containedin the fine pores.

For the produced secondary particles ε, which were produced by thismethod, an observation of the scanning electron microscope was conductedafter conducted the heat treatment under various kinds of conditions.

From these particles, the abrasive grain ζ related to the presentinvention is produced. This abrasive grain ζ is the porous particlematerial in which a large number of fine particles for the cutting edgescontain gaps between primary particles bonded to each other, and alsothe particles for the cutting edges are formed by growing the primaryparticles with the heat treatment.

The strength of compression failure of this abrasive ζ was 124.2 MPa,and size of the particles for the cutting edges was 1.2 μm.

Polyurethane resin paint was mixed with the produced abrasive grain ζ inwhich the volume ratio becomes 35 percent of its volume and copperpowder of the average particles diameter of 3 μm. After that, themixture was produced by mixture and agitation of particles and resin for15 minutes by the agitator at the rotation speed of 60 rpm.

At this point, it is possible to add blowing agent to form independentbubble depending on need.

The mixture was poured into a circular form metal mold (450 mmΦ), thenit was hardened with the heat treatment at 120° C. for 10 hours, andthen the polishing pad was produced. The produced polishing pad wasattached to the surface platen after cutting it into a specified size,and then polishing processing was conducted for a silicon wafer whichwas grinded by a grinding wheel of about #2000 in advance.

As a result, the mirror surface, which does not have any scratch andlower than 20 nm of the maximum height for the surface roughness Ry, wasproduced in 10 minutes of the processing. Even though polishing of 20sheets of the silicon wafers were conducted continuously; processingefficiency and processing surface roughness were not deteriorated.

At this point, FIG. 19 shows an example of attaching the polishing pad,which is related to the present invention used for the testing example2, to the polishing apparatus for processing silicon as a model.

For the FIG. 19, the sign 1 is a work piece, a silicon wafer. Thesilicon wafer is mounted on a rotation part 10. The silicon wafer isrotated by the rotation of this rotation part 10. An undersurface of thesilicon wafer is also polished by contacting to the polishing pad 2related to the present invention, which was mounted on a platen 20(lapping film can be mounted instead of the polishing pad) in accordancewith upward and downward movement of the rotation part 10. Moreover, thewhole undersurface of the silicon wafer 1 is polished equally becausethe surface platen 20 rotates.

Testing Example 3

Using silica abrasive grain as the polishing pad was considered. Thestrength of compression failure of the silica abrasive grain was 18.5MPa and size of particles for the cutting edges was 0.2 μm, which wasproduced by the heat treatment under different condition from testingexample 2 for secondly particles ε.

Polishing pad was produced by use of the silica abrasive grain under thesame condition as testing example 2, and a processing test of a siliconwafer by using the grinding cloth was conducted as well. As a result, amirror surface, which has lower than 20 nm of maximum height for surfaceroughness Ry, was produced in 15 minutes of a processing time. However,after the processing was conducted continuously, the processingefficiency was declined gradually. When processing 5 sheets of thesilicon wafer, 30 percent of the processing efficiency was declinedcompared to the beginning of the processing, although the processingsurface roughness is the same level, and then it became impossible toprocess after the 15^(th) sheets of the silicon wafer.

Testing Example 4

Phenol aldehyde resin was mixed with the abrasive grain related to thepresent invention comprising the average particle diameter of 30 μm andthe strength of compression failure of 124.2 MPa as the same as used inthe testing example 2 in which the volume ratio becomes 45 percent ofits volume at last and nickel powder (reinforcement material) of theaverage particle diameter of 3 μm becomes 15 percent of its volume.After that, the mixture was produced by mixture and agitation the powderand resin for 15 minutes by the agitator at the rotation speed of 60rpm.

The mixture was poured into the metal mold, and a grinding wheel wasproduced by a hardening treatment for about 5 hours at the temperatureof 150° C. with pressuring.

After the produced grinding wheel was attached to vertical axis ofInfeed Grinding Machine, and grinded a lapping finished silicon wafer,mirror surface, which has lower than 30 nm of the maximum height for thesurface roughness Ry, was produced in one minutes of a processing time.After grinding for 20 sheets of silicon wafer continuously, grindingburn was not generated, and deterioration of the grinding efficiency andsurface roughness were not identified.

Testing Example 5

A polishing processing test for a silicon wafer was conducted under thecondition, which used the abrasive grain related to the presentinvention comprising the same strength for compression failure 18.5 MPaas used in the testing example 3, and beside that it was used the sameconditions as the testing example 4 to manufacture the grinding wheel.

As a result, a mirror surface, which has lower than 30 nm of the maximumheight for the surface Ry, was produced in one minute of processingtime. However, after processing was conducted continuously, the grindingburn was generated on the surface of the silicon wafer when 10^(th)sheets of silicon wafer were processed.

As explaining above, the abrasive grain of the present inventionconsists of the porous particle material. The porous material iscomposed of a large number of fine particles for the cutting edges,which contain gaps between primary particles bonded to each other. Theparticles for the cutting edges are produced by growing the primaryparticles in the secondary particles, which were formed by the cohesionof a large number of the primary particles, with the heat treatment atthe temperature of forming the necks at the bonding point among theprimary particles. Therefore, when using as the abrasive grain, at leastthe part, which is contacted to the gap of the particles for the cuttingedges on the processing surface, woks as the cutting edges.

The particles for the cutting edges are also projected newly on theprocessing surface automatically while the part comprising the particlesfor the cutting edges is lost by the wear of the particles for thecutting edges. Moreover, the bonding strength among the particles forthe cutting edges is optimized, so that it is possible to maintain theexcellent quality and have the stable processing for the long processingtime very effectively.

The polishing tool of the present invention also has the high processingefficiency, the high quality of processing, and the long operating life.

The polishing apparatus of the present invention also has the highpolishing efficiency, the high quality of polishing, and the longoperating life for the polishing tool.

According to the producing method of the abrasive grain of the presentinvention, when processing of polishing, the cutting edges are alwaysgenerated voluntarily for the abrasive grain. Therefore, it makes easyto eliminate the waste of the polishing, and it is certainly possible toproduce the abrasive grain, which can polish very effectively withmaintaining the excellent quality, stably with the high productivity.

According to the producing method of the polishing tool of the presentinvention, it is also possible to produce the long operating lifepolishing tool, which can polish very effectively with maintainingexcellent quality.

According to the producing method of the grinding wheel of the presentinvention, it is possible to produce the grinding wheel, which can grindvery effectively with maintaining the excellent quality.

1. An abrasive grain, comprising: a plurality of primary particles; anda secondary particle comprising a plurality of primary particles,wherein in the secondary particle: the plurality of primary particlesare bonded through neck portions between a portion of at least one ofthe primary particles and another of the primary particles, theplurality of primary particles form grinding surfaces configured toperform grinding, and each of the neck portions has a hyperboloid of onesheet shape, comprises a comprising materials from the adjacent primaryparticles.
 2. The abrasive grain according to claim 1, wherein a surfaceof the secondary particles functions as a grinding surface to performgrinding, and the portion of the plurality of primary particles, whichare bonded to form the secondary particle, is configured to break awayat the neck portion.
 3. The abrasive grain according to claim 1, whereinthe secondary particle has a strength of compression failure which ismore than 1 MPa and less than 500 MPa.
 4. The abrasive grain accordingto claim 1, wherein the secondary particle has pores specific surfacearea which is more than 18000 cm²/cm³ and less than 700000 cm²/cm³. 5.The abrasive grain according to claim 1, wherein an average particlediameter of said primary particles is smaller than 5 μm.
 6. The abrasivegrain according to claim 1, wherein a binder is not included to bond theprimary particles.
 7. The abrasive grain according to claim 1, whereinthe secondary particle has a strength of compression failure which ismore than 20 MPa and less than 300 MPa.
 8. The abrasive grain accordingto claim 1, wherein the secondary particle has a pores specific surfacearea which is more than 100000 cm²/cm³ and less than 300000 cm²/cm³.